A wide variety of IMDs that employ electronic circuitry for providing electrical stimulation of body tissue and/or monitoring a physiologic condition are known in the art. A number of IMDs of various types are known in the art for delivering electrical stimulating pulses to selected body tissue and typically comprise an implantable pulse generator (IPG) for generating the stimulating pulses under prescribed conditions and at least one lead bearing a stimulation electrode for delivering the stimulating pulses to the selected tissue. For example, cardiac pacemakers and implantable cardioverter-defibrillators (ICDs) have been developed for maintaining a desired heart rate during episodes of bradycardia or for applying cardioversion or defibrillation therapies to the heart upon detection of serious arrhythmias. Other nerve, brain, muscle and organ tissue stimulating medical devices are also known for treating a variety of conditions.
Currently available IMD IPGs including ICD and cardiac pacemaker IPGs are typically formed having a metallic housing that is hermetically sealed and, therefore, is impervious to body fluids, a header or connector assembly mounted to the housing for making electrical and mechanical connection with one or more leads, and possess telemetry capabilities for communicating with external devices. Over the past 20 years, ICD IPGs have evolved, as described in some detail in commonly assigned U.S. Pat. No. 5,265,588, from relatively bulky, crude, and short-lived ICD IPGs simply providing high energy defibrillation shocks to complex, long-lived, and miniaturized ICD IPGs providing a wide variety of pacing, cardioversion and defibrillation therapies. Numerous improvements have been made in cardioversion/defibrillation leads and electrodes that have enabled the cardioversion/defibrillation energy to be precisely delivered about selected upper and lower heart chambers and thereby dramatically reducing the delivered shock energy required to cardiovert or defibrillate the heart chamber. Moreover, the high voltage output circuitry has been improved in many respects to provide monophasic, biphasic, or multi-phase cardioversion/defibrillation shock or pulse waveforms that are efficacious, sometimes with particular combinations of cardioversion/defibrillation electrodes, in lowering the required shock energy to cardiovert or defibrillate the heart.
Such ICD IPGs need to be small enough to be comfortably implanted subcutaneously without being unduly uncomfortable to the patient or cosmetically apparent. The first implanted automatic implantable defibrillator (AID) IPG housing disclosed in U.S. Pat. No. 4,254,775 was very large and had to be implanted in a patient's abdominal region. Since that time, the ICD IPGs have been reduced in size while their complexity has been vastly increased. Battery energy requirements for powering both the low voltage integrated circuits (ICs) and for providing the cardioversion/defibrillation shocks have been reduced while battery energy density has been increased and battery configuration made more conforming to the interior space of the ICD IPG housing. Miniaturized, flat high voltage output capacitors that can be shaped to fit the allocated housing space and miniaturized high voltage switching components have been developed and employed. All of these improvements, together with the above-mentioned cardioversion/defibrillation improvements have contributed to a significant reduction in the volume of the ICD IPG housing without sacrificing longevity and capabilities.
Similar improvements in reducing housing volume have been made in other IMD IPGs, particularly implantable cardiac pacemakers, nerve stimulators and monitors, over the same time period. Remote programming and interrogation of IMD operating modes and parameters has been implemented in the above-described IMDs employing uplink (from the IMD) and downlink (to the IMD) telemetry transmissions between an RF transceiver within the IMD and an external transceiver of an external “programnmer”. Such programmers are used to program the IMD by downlink telemetry transmission of commands that are received and stored in memory incorporated within the IMD that change an operating mode or parameter value governing a function performed by the IMD.
Both non-physiologic and physiologic data (collectively referred to herein as “patient data”) can be transmitted by uplink RF telemetry from the IMD to the external programmer. The physiologic data typically includes stored and real time sampled physiologic signals, e.g., intracardiac electrocardiogram amplitude values, and sensor output signals. The non-physiologic patient data includes currently programmed device operating modes and parameter values, battery condition, device ID, patient ID, implantation dates, device programming history, real time event markers, and the like. In the context of implantable pacemakers and ICDs, such patient data includes programmed sense amplifier sensitivity, pacing or cardioversion pulse amplitude, energy, and pulse width, pacing or cardioversion lead impedance, and accumulated statistics related to device performance, e.g., data related to detected arrhythmia episodes and applied therapies.
The RF telemetry transmission system that evolved into current common usage relies upon magnetic field coupling through the patient's skin of an ND IPG antenna with a closely spaced programmer antenna. Low amplitude magnetic fields are generated by current oscillating in an LC circuit of an RF telemetry antenna of the ND or programmer in a transmitting mode. The currents induced in the closely spaced RF telemetry antenna of the programmer or ND are detected and decoded in a receiving mode. Short duration bursts of the carrier frequency are transmitted in a variety of telemetry transmission formats. In the MEDTRONIC® product line, the RF carrier frequency is set at 175 kHz, and the IMD RF telemetry antenna located within the IMD housing is typically formed of coiled wire wound about a bulky ferrite core.
Apart from the bulk of the antenna taking up valuable space within the IMD housing, there are a number of other limitations in the current MEDTRONIC® telemetry system employing the 175 kHz carrier frequency. First, using a ferrite core, wire coil, RF telemetry antenna results in a very low radiation efficiency because of feed impedance mismatch and ohmic losses and a radiation intensity attenuated proportionally to at least the fourth power of distance (in contrast to other radiation systems which have radiation intensity attenuated proportionally to square of distance). These characteristics require that the implantable medical device be implanted just under the patient's skin and preferably oriented with the RF telemetry antenna closest to the patient's skin so that magnetic field coupling is provided. To ensure that the data transfer is reliable, it is necessary for the patient to remain still and for the medical professional to steadily hold the RF programmer head against the patient's skin over the IMD for the duration of the transmission. The time delays between downlink telemetry transmissions depend upon the user of the programmer, and there is a chance that the programmer head will not be held steady. If the uplink telemetry transmission link is interrupted by a gross movement, it is necessary to restart and repeat the uplink telemetry transmission.
Secondly, the RF telemetry data transmission rate is limited employing a 175 kHz carrier frequency. As device operating and monitoring capabilities multiply, it is desirable to be able to transmit out ever increasing volumes of data in real time or in as short a transmission time as possible with high reliability and immunity to spurious noise in an uplink telemetry transmission from an IMD, it is desirable to limit the current drain from the implanted battery as much as possible, simply to prolong IMD longevity.
As a result of these considerations, IMD-programnmer RF telemetry schemes have been proposed having the objectives of eliminating the bulky antenna within the IMD housing to conserve space, eliminating the need for the close, steady magnetic field coupling to enable uplink and downlink telemetry transmission at a greater distance, increasing the data transmission rate, and minimizing battery consumption. It has been proposed to replace the ferrite core, wire coil, RF telemetry antenna in the IMD with an antenna that can be located outside the hermetically sealed enclosure and to employ higher carrier frequencies and data transmission rates. It is suggested that the RF telemetry antenna may be located in the IPG header in U.S. Pat. No. 5,342,408. A further U.S. Pat. No. 4,681,111 suggests the use of a stub antenna associated with the header as the IMD RF telemetry antenna for high carrier frequencies of up to 200 MHz and employing phase shift keying (PSK) modulation. The elimination of the need for a VCO and a bit rate on the order of 2-5% of the carrier frequency or 3.3-10 times the conventional bit rate are alleged. In commonly assigned U.S. Pat. No. 5,861,019, it is proposed that the telemetry antenna be formed as a microstrip patch antenna that is conformal with the exterior housing of the IMD. The uses of the 204-216 MHz band and the 902-908 MHz ISM band have been proposed in U.S. Pat. Nos. 5,944,659 and 5,767,791 for telemetry transmissions between externally worn patient monitors and fixed location networked transceivers for ambulatory patient monitoring in a hospital context.
In order to fit within a minimal space, all electronic circuits of the IMD circuitry are preferably formed as ICs, but implementation of RF telemetry circuitry has required use of discrete capacitors and inductors mounted along with the RF IC to a circuit board. The use of such discrete inductors and capacitors is not eliminated, but is actually increased, in the above-described high frequency, high data transmission rate telemetry transceiver circuitry. The relocation of the bulky RF telemetry antenna to a less critical area outside the hermetically sealed IMD housing reduces space requirements. However, the discrete inductor and capacitor components mounted with one or more IC chip to an RF IC module substrate render the RF module unduly large such that it occupies a substantial portion of the volume within the IMD housing.